D2D communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite such as WiFi Direct. These systems or technologies operate in unlicensed spectrum.
Recently, D2D communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment.
It is suggested that such a D2D communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for D2D purposes. Allocating dedicated spectrum for D2D purposes is a less likely alternative as spectrum is a scarce resource and (dynamic) sharing between the D2D services and cellular services is more flexible and provides higher spectrum efficiency.
Devices that want/wish to communicate, or even just discover each other, typically need to transmit various forms of control signaling. One example of such control signaling is the discovery signal; which may include a full message e.g. a synchronization message or a beacon; which at least carries some form of identity and is transmitted by a device that wants/wishes to be discoverable by other devices. Other devices may scan for the discovery signals. Once they have detected the discovery signal, they may take the appropriate action, for example to try to initiate a connection setup with the device transmitting the discovery message.
Multiple discovery signals from different user equipments (UEs), being an example of a device, are multiplexed on the same radio resources in a combination of Time Division Multiplexing (TDM), Frequency Division Multiplexing (FDM) and/or Code Division Multiplexing (CDM). Even though details are not agreed yet at the standardization meeting e.g. 3GPP (Third Generation Partnership Project) or IEEE (Institute of Electrical and Electronics Engineers), it is likely that discovery signals be multiplexed on specific subframes occurring at known (or signaled) positions in a radio frame. Similarly to the discovery signals, it is envisioned that UEs transmit channels for data and/or control information.
D2D discovery may be viewed as an application where UEs sense each other's presence based on detection of periodically broadcasted discovery messages. Each UE transmits one or more discovery messages within configured resources also called a discovery resource pool. The discovery resource pools may be different across cells and they be offset in time due also to synchronization differences between cells. Usually a cell is served by a radio base station or a eNB or eNodeB. This implies that the UEs in one cell may need to synchronize to the resource pools provided by other cells in order to detect the associated discovery messages. One way of providing this synchronization is to associate pools to synchronization signals that are particularly suitable for the purpose.
Resource allocation for discovery includes a discovery period that may even be several seconds long. Within the discovery period, each discovery pool generally spans a small fraction of resources. The discovery resources should be almost contiguous in time for energy efficiency reasons. The discovery resource pool may only span a few tens or hundreds of milliseconds within the discovery period.
Traditional communication in terrestrial radio networks is via links between User Equipments (UEs) and base stations. However, when two UEs are in the vicinity of each other, then direct D2D communication may be considered. Such communication may be dependent on synchronization information from either a base station or a different node such as a cluster head (CH) (a UE acting as synchronization source) providing local synchronization information, or a UE enabled to relay synchronization information from a different synchronization source. The synchronization source from eNB/CH is used for intra-cell/cluster communication. The relayed synchronization signal is used for inter-cell/cluster communication. An illustration of synchronization source from different node is shown in FIG. 1.
Two cells; Cell 1 and Cell 2; and a cluster (Cluster 1) are shown. Each cell is served by a eNB or base station. Cell 1 is shown including three UEs and a eNB. Cell 2 is shown including one UE and a eNB and Cluster 1 is shown including 3 UEs. One of the UEs in Cluster 1 acts as cluster head (CH). Synchronization signals shown in continuous lines are broadcasted by eNBs or base stations. In Cluster 1, CH transmits synchronization signals (dot-dash). Synchronization signals (dotted lines) between UEs (D2Ds) are also shown.
For in-coverage D2D scenarios, the synchronization reference is provided by eNB or base station. So in this case, the D2D resource pool is signaled by eNB to indicate the resource used for D2D. While for out of coverage D2D scenarios, the synchronization reference is provided by CH.
The signal design of D2D Synchronization Signals (D2DSSs) is under discussion in 3GPP. One solution includes differentiating D2DSSs into two sets, one intended to support D2D discovery and the other set intended to support D2D communication.
Synchronization signals are typically periodically transmitted. The reason is that oscillators tend to drift from their nominal value and periodic corrections based on received synchronization are needed to maintain relative synchronization. Typical synchronization periodicity may be from a few milliseconds to tens or hundreds of milliseconds. FIG. 2 illustrates a discovery period including discovery resources within a pool and periodic D2DSS resources used for discovery. A D2DSS is transmitted periodically within the discovery period. The discovery resources comprise set of time/frequency radio resources. UEs may transmit discovery messages or signals only within such pool, where each discovery message/signal typically occupies a small fraction of the pool resources. Thus, several transmitters may share the same pool by using different resources in the pool. Receivers on the other hand need to monitor the whole pool and attempt detection of any discovery message/signal potentially transmitted in the pool. The discovery message/signal carries some identity associated to the transmitter. Once a discovery message is detected, the receiver becomes aware of the proximity of the associated transmitter.
The periodic synchronization signals may be viewed as fixed overhead that prevent reuse of D2DSS resources for other purpose. There is therefore a need to minimize the overhead. With deployment of D2D on cellular spectrum minimizing the overhead associated to D2D is advantageous. Further, D2D signal transmissions may drain battery life of D2D capable UEs especially if idle UEs transmit such D2DSS signals in a periodic fashion.